P1406: TIMING OF EXPOSURE TO INFECTIOUS MICROBIOTA DETERMINES HSC FITNESS
Authors: J. Jayarajan, M. Ball, D. Shah, S. Lux, F. Grünschläger, E. Rodriguez-Correa, F. Fotopoulou, J. Knoch, A.-M. Mikecin, S. Haas, K. Schmidt, S. Rosshart, M. Milsom
Journal: HemaSphere (2022)

Background: Chronic infections in mice have been shown to lead to an increased proliferation of the hematopoietic stem and progenitor cell (HSPCs) compartment and a subsequent loss of stem cell function. However, many of these infection-models involve challenging mice with a very high dose of purified pathogen administered via a non-physiologic route, thereby not recapitulating a natural infection/colonization process. Recent studies from Rosshart et al (PMID: 31371577) has led to the generation of so-called “wildling” mice that carry a full spectrum of microbiota/pathogens found in wild mice, but on a full C57BL/6 genetic background. Aims: To compare the effects of horizontal (co-housing-mediated) versus vertical transmission of wild microbiota and pathogens into C57BL/6 mice with a focus on generation of mature cells from HSPCs. Methods: SPF C57BL/6 mice were co-housed with wildling mice for a period of 3 weeks. 6 months from the start of co-housing, the bone marrow of co-housed mice and age-matched wildling mice was analyzed using flow cytometry, and HSC function was assessed by competitive bone marrow transplantation assays. Additionally, BrdU labelling experiments were performed to assess HSC proliferation rate. Mice housed under SPF hygiene status were used as controls for all experiments. Results: 3 weeks of co-housing resulted in a robust transfer of most viral and bacterial pathogens, and parasites such as mites into co-housed mice. These co-housed mice showed sustained elevated peripheral blood counts in response to the transmitted microbiota. However, neither the co-housed mice nor the wildling mice exhibited any changes in HSPC proliferation or numbers in the bone marrow. Competitive bone marrow transplantation using bone marrow from wildling mice that have neonatal microbial colonization, showed no defect in long-term HSC engraftment suggesting uncompromised HSC function. In contrast, bone marrow from adult co-housed mice showed a significant reduction in long-term engraftment suggesting a reduction in HSC function even though no changes in HSC proliferation were perceivable at the time point of analysis. Summary/Conclusion: Our study has shown that horizontal transmission of microbiota/pathogens to immunologically naïve adult mice leads to a reduction in HSC function similar to that shown by other groups with more severe infection models. However, the lack of reduction in HSC function in wildling mice suggests that neonatal exposure and colonization of pathogens maintains HSC fitness in adults. This suggests there is a context-specific window of opportunity for infection-mediated functional attrition of HSCs. We are currently analysing scRNA-seq data from bone marrow hematopoietic and niche cells to dissect the molecular differences in steady state hematopoiesis of mice subject to horizontal versus vertical transfer of pathogens/microbiota, to better understand why the HSC function might be compromised in co-housed mice but not in wildling mice.